1. Field of the Invention
The present invention relates to a brake system of an engine.
2. Description of the Related Art
In the past, brake boosters have been used to increase the braking force. Such a brake booster basically is provided with a first chamber and a second chamber separated by a power piston. The first chamber is connected to a vacuum takeout port provided at a vacuum generating region in the engine intake passage. When the braking action is not being performed, the second chamber is connected with the first chamber, therefore at this time a vacuum is led from the vacuum takeout port into the first and second chambers chamber. On the other hand, when the brake pedal is depressed, the connection between the first chamber and the second chamber is broken and, simultaneously, the second chamber is opened to the atmosphere. Therefore, at this time, a large pressure difference is generated between the first chamber and the second chamber. The power piston is driven by this pressure difference whereby a large braking force is generated.
Next, when the brake pedal is released, the second chamber is closed off from the atmosphere and the second chamber is once again connected to the first one. As a result, the vacuum which is generated in the intake passage is once again led to the second chamber from the vacuum takeout port. In other words, at this time, air is rapidly ejected from the vacuum takeout port into the intake passage. In internal combustion engines where an air flow meter is provided at the inlet of the intake passage and the amount of fuel injection is calculated based on the amount of intake air detected by this air flow meter, if air is supplied from the vacuum takeout port in the intake passage downstream of this air flow meter, the air-fuel ratio will fluctuate. At this time, if, as explained above, air is rapidly ejected from the vacuum takeout port, the air-fuel ratio will widely fluctuate.
Therefore, to restrain the fluctuation of the air-fuel ratio due to the action of the air ejected from the vacuum takeout port when the brake pedal is released, there is known an internal combustion engine which increases the amount of the fuel injection when the brake pedal is released (see Japanese Unexamined Patent Publication (Kokai) No. 7-19086) and an internal combustion engine which ejects air a little at a time from the vacuum takeout port when the vehicle speed is low when the brake pedal is released (see Japanese Unexamined Patent Publication (Kokai) No. 7-119511).
In practice, however, it is difficult to increase the injection of fuel by the necessary amount in complete synchronization with the action of ejection of air from the vacuum takeout port. Therefore, in the internal combustion engine disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-19086, there is the problem that the air-fuel ratio fluctuates by a considerable extent. Further, in the internal combustion engine disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-119511, there is the problem that when the vehicle speed is high, air is rapidly ejected from the vacuum takeout port and therefore the air-fuel ratio fluctuates by a considerable degree even in such an engine.